An active-matrix-addressed liquid crystal display device generally includes a substrate on which thin-film transistors (which will also be referred to herein as “TFTs”) are provided as switching elements for respective pixels (such a substrate will be referred to herein as a “TFT substrate”), a counter substrate on which a counter electrode, color filters and other members are arranged, a liquid crystal layer which is interposed between the TFT substrate and the counter substrate, and a pair of electrodes to apply a voltage to the liquid crystal layer.
Various modes of operation have been proposed and adopted for active-matrix-addressed liquid crystal display devices according to their intended application. Examples of those modes of operation include a TN (Twisted Nematic) mode, a VA (Vertical Alignment) mode, an IPS (In-Plane-Switching) mode and an FFS (Fringe Field Switching) mode.
Among these modes, the TN and VA modes are longitudinal electric field modes in which a pair of electrodes that face each other with a liquid crystal layer interposed between them apply an electric field to liquid crystal molecules. On the other hand, the IPS and FFS modes are lateral electric field modes in which a pair of electrodes is provided for one substrate to apply an electric field to liquid crystal molecules parallel to the surface of the substrate (i.e., laterally). According to the lateral electric field method, liquid crystal molecules do not rise with respect to the substrate, and therefore, a wider viewing angle can be achieved than in the longitudinal electric field method, which is beneficial.
Among various modes of operation by the lateral electric field method, in an IPS mode liquid crystal display device, a pair of comb electrodes are formed on a TFT substrate by patterning a metal film, and therefore, the transmittance and aperture ratio will decrease, which is a problem. On the other hand, in an FFS mode liquid crystal display device, the electrodes to be formed on the TFT substrate are transparent, and therefore, the aperture ratio and transmittance can be increased.
FFS mode liquid crystal display devices are disclosed in Patent Documents Nos. 1 and 2, for example.
On the TFT substrate of these display devices, a common electrode and a pixel electrode are arranged over each TFT with an insulating film interposed between them. Among these electrodes, a hole is cut as a slit through the electrode which is located closer to the liquid crystal layer (e.g., the pixel electrode). As a result, generated is an electric field which is represented by electric lines of force that are emitted from the pixel electrode, pass through the liquid crystal layer and the slit hole, and then reach the common electrode. This electric field has a lateral component with respect to the liquid crystal layer. Consequently, a lateral electric field can be applied to the liquid crystal layer.
Recently, people have proposed that an oxide semiconductor be used as a material for the active layer of a TFT instead of a silicon semiconductor. Such a TFT will be referred to herein as an “oxide semiconductor TFT”. Since an oxide semiconductor has higher mobility than amorphous silicon, the oxide semiconductor TFT can operate at higher speeds than an amorphous silicon TFT. For example, Patent Document No. 3 discloses an active-matrix-addressed liquid crystal display device which uses an oxide semiconductor TFT as a switching element.